Study On Improving The Quality Of Miscanthus Pyrolysis Oil By Combination Of Torrefaction And Dual Catalyst

In order to meet the growing global energy demand and achieve the goal of sustainable energy development,biomass,as a kind of renewable carbon energy with rich reserves in nature,is of great significance for its high-value and resource utilization.Bio-oil produced by biomass pyrolysis is considered as a potential choice to achieve energy security,which is of positive significance to the environment,economy and society.However,the complex composition,high moisture and oxygen content,strong acidity and poor stability of bio-oil from direct pyrolysis of biomass seriously restrict its development and application.Therefore,this paper aims to improve the quality of bio-oil by reducing the content of oxygenates in bio-oil and increasing the yield of aromatic hydrocarbons in bio-oil.Miscanthus was selected as the raw material of biomass.The torrefaction pretreatment was performed first,and then the catalytic fast pyrolysis experiments were carried out.The effect of torrefaction on the physicochemical properties of Miscanthus was studied.The original and torrefied Miscanthus were characterized by proximate and ultimate analyses,Fourier transform infrared spectroscopy(FTIR)analysis,and Scanning Electron Microscope(SEM)analysis.It was found that the number of oxygen-containing functional groups and oxygen content were significantly decreased,and the fixed carbon content and HHV value were increased after Miscanthus was by torrefied.When torrefaction temperature was 280℃,the contents of C and O in Miscanthus were 54.89%and 28.68%,respectively,and the HHV value was 22 MJ/kg.Therefore,torrefaction is beneficial to the improvement of fuel characteristics and physicochemical properties of Miscanthus,which is conducive to the subsequent heat conversion and utilization.The effects of torrefaction temperature and catalysts(ZSM-5,CaO and MoO3)on the pyrolysis characteristics of Miscanthus were studied by thermogravimetric analyzer.The results showed that the initial pyrolysis temperature increased,the weight loss decreased,and the thermal stability improved obviously.With the increase of torrefaction temperature,the maximum weight loss rate was decreased,but the corresponding pyrolysis temperature did not change significantly.After the addition of catalyst,the pyrolysis process of Miscanthus before and after torrefaction was roughly the same with the non-catalytic pyrolysis,but the maximum weight loss rate decreased significantly.According to kinetic calculation,the pyrolysis activation energy of Miscanthus increased after torrefaction,but decreased during catalytic pyrolysis process.The pyrolysis experiments of Miscanthus before and after torrefaction were carried out by Py-GC/MS.The effects of torrefaction temperature,pyrolysis temperature and catalyst types(ZSM-5,CaO,MoO3,dual catalysts CaO/ZSM-5 and MoO3/ZSM-5)on the pyrolysis oil components of Miscanthus were studied.The results showed that with the increase of torrefaction temperature,the oxygen-containing compounds in the pyrolysis oil of Miscanthus gradually decreased.After ZSM-5 catalyst was added,the main components of the pyrolysis oil changed from oxygen-containing compounds to aromatic hydrocarbons.The selectivities of benzene,toluene and xylene in bio-oil(bio-BTX)increased with increasing torrefaction temperature up to 56.63%.Compared with ZSM-5 alone,the addition of CaO and MoO3 enhanced the deoxidation and olefins aromatization reactions,thus further promoting bio-BTX formation.The MoO3/ZSM-5 significantly improved the selectivity of bio-BTX,reaching the highest selectivity of 70.69%and a yield of 51.07%.Therefore,the Py-GC/MS pyrolysis experiment of Miscanthus proved that the combination of torrefaction pretreatment and dual catalyst catalytic pyrolysis could significantly increase the yield of aromatic hydrocarbons and reduce the content of oxygenated compounds in the pyrolysis oil of Miscanthus,thus the quality of bio-oil was improved.The effects of pyrolysis temperature,torrefaction temperature,ZSM-5,MoO3 and MoO3/ZSM-5 catalyst on the yield and the composition of three phase pyrolysis products(char,bio-oil and gas)were studied in a fixed-bed pyrolysis system.The results showed that the yield of pyrolysis oil from torrefied Miscanthus was the highest(29.23%)when the pyrolysis temperature was 550℃;the pyrolysis oil yield decreased,the oxygen compounds in the oil decreased significantly,and the content of aromatic hydrocarbons increased after torrefaction.In the pyrolysis experiment of Miscanthus catalyzed by MoO3/ZSM-5,aromatic hydrocarbons were further enriched into bio-oil,and the yield of aromatic hydrocarbons was as high as 91.38%,while the content of oxygen compounds was only 6.27%.The quality of pyrolysis oil was significantly improved.Therefore,the combination of torrefaction pretreatment and dual catalyst catalytic fast pyrolysis has a great effect on improving the quality of biomass pyrolysis oil.